Method for manufacture of paper and a paper machine

ABSTRACT

The invention concerns a method for manufacture of paper, most appropriately for porous paper for color powder printing. In the method paper is manufactured by means of a paper machine. In the method the paper stock is fed out of the headbox ( 100 ) into the wire part ( 200 ), in which wire part ( 200 ) water is drained out of the paper web in both directions. In the method the paper web (W) is passed from the wire part ( 200 ) into the press section ( 300 ) in order to press water out of the paper web (W), and after the press section ( 300 ), the paper web (W) is dried in the dryer section ( 400 ) and coated/pigmented in the coating section ( 500 ), dried in an after-dryer section ( 600 ) and calendered in a calender ( 600 ), and reeled in a reel-up ( 800 ). The paper web (W) is made of layers in the Z-direction so that the desired distributions of additives and fillers are obtained in the different layers in the Z-direction of the paper web (W), and the paper web (W) is calendered in at least one calendering nip, which maintains or at least substantially retains the porosity of the web preceding the calendering. The invention also concerns a paper machine, most appropriately for manufacture of glossy and porous paper for color powder based printing. The paper machine comprises a headbox ( 100 ), a wire part ( 200 ), a press section ( 300 ), a dryer section ( 400 ), a coating section ( 500 ), an after-dryer section ( 600 ), a calender ( 700 ), and a reel-up ( 800 ). The headbox ( 100 ) and the wire part ( 200 ) have been formed so that the desired layers with different compositions in the Z-direction are provided in the paper, and the calender ( 700 ) is a calender device that maintains or at least substantially retains the porosity of the paper web (W) preceding the calendering.

FIELD OF THE INVENTION

The present invention relates to a paper machine including a headbox, awire part in which water is drained from a paper web, a press sectionarranged afer the wire section in the running direction of the papermachine for pressing water out of the web, a dryer section arrangedafter the press section, a coating section arranged after the dryersection, an after dryer arranged after the coating section, a calendersection arranged after the after dryer and a reel up arranged after thecalender section. The present invention also relates to a method ofmaking paper in a paper machine of the type described above.

BACKGROUND OF THE INVENTION

Paper grades manufactured by means of prior-art methods and papermachines do not meet all present-day or future needs in respect of papergrades. One important need is glossy porous paper, which is well suitedfor colour-powder based printing. This need arises, among other things,from the fact that, along with increasing speeds and improved printingquality of colour copiers, in colour-powder based printing, colourcopiers have started competing with sheet-offset printing and, on theother hand, this need also arises from the fact that, in black/whiteprinting, digital processing of image and laser technology have improvedthe quality of the picture to the level of offset printing. One problemin printing with a colour copier is inadequate gloss of the paper to beused. Coated paper is not directly suitable for use in sheet-feed colourcopiers, but one of the problems is a so-called glass-sheet effectbetween two sheets, i.e. the sheets tend to adhere to each other. Infriction-based feeders in colour copiers, coated papers do not operateat all, and also in vacuumbased feeders they operate poorly. Thus, incolour-powder printing, such a coated or compacted paper is needed asdoes not adhere to the colour powder fixing unit and in whose use saidproblem of glass-sheet effect does not occur. It has also been a problemthat humidity may remain in the interior of compacted paper when thepaper is dried from both sides.

Thus, in actual fact, there is no method or paper machine suited for themanufacture of papers of this type, and of the paper grades currentlyavailable, usually a supercalendered fine paper is used, but it is aproblem of this paper grade that it is not sufficiently glossy.

As is known from the prior art, said paper grades are manufactured inpaper machines in which the headbox is, for example, a hydraulicheadbox, for example SymFlo™ or some other conventional headbox of thesame type. The wire part that is used is a fourdrinier wire or a hybridwire part, for example SymFormer™, in which there are a fourdrinier wireand an upper-wire unit. Also, gap formers have been used in themanufacture of paper grades of different types. From the prior art, amethod is also known for application of additives, fillers and chemicalsas layers. This method is described in the patent EP 0 651 092. As apress section, presses of many types are used, usually roll presses.From the prior art, different extended-nip press solutions are alsoknown, which have, however, not been applied to the manufacture ofpapers used for colour-powder based printing. The dryer sections havebeen made of conventional dryer sections which make use of single-wireor twin-wire draw and in which the drying takes place primarily ascylinder drying. Impingement drying is known from a number of differentpatent publications, but industrial applications are not in operation asyet. On the contrary, some use has been made of air drying arranged bymeans of the principle of infra drying or airborne-web drying. As sizepresses, size presses of many different types are used, for example tubsize presses or solutions of the type of the applicant's SymSizer. Thecalender has, as a rule, been a soft calender with one or two nips andcombinations formed out of them. Also, supercalendering has been usedfor the manufacture of the final product. The reel-up has been asuitable reel-up. Paper machines of the types described above andcomponent units of said machines have been described, for example, inthe following published patent applications and patents: FI 75,377, FI83,540, FI 98,540, U.S. Pat. No. 4,075,056, EP 0 770 727, FI 98,387, FI901967, and FI 924960. By means of these machines, a paper grade wellsuited for colour-powder based printing has, however, not been achieved,owing to the above problems.

The prior art does not know a suitable paper machine or an applicablemethod for manufacture of paper that has, first, copying paperproperties and good gloss and suitable porosity for colour-powderprinting. Further, among traditional properties of copying paper,important properties are, among other things, properties of electriccharge and resistivity and dimensional stability. It is, however,expected that the need of paper grades of this type will increase in thenear future, so that a method and a paper machine are needed formanufacture of such papers.

OBJECTS AND SUMMARY OF THE INVENTION

Thus, the object of the present invention is to provide a method and apaper machine by whose means it is possible to manufacture especially apaper of this type with suitable gloss and suitable porosity.

In accordance with the invention, the paper web is formed as layers inthe Z-direction so that the desired distributions of additives andfillers are obtained in the different layers in the Z-direction in theweb. The web is calendered in at least one calendering nip, whichmaintains or at least substantially retains the porosity of the webpreceding the calendering. Favourably, the web is calendered in a shoecalender which comprises an extended calendering nip. Successivecalendering operations can be carried out in the same calendering deviceor in separate successive devices.

In accordance with the invention, the headbox and the wire part of thepaper machine have been formed so that the desired composition of layersis obtained for the paper in the Z-direction. The calender is a calenderdevice that maintains or at least substantially retains the porosity ofthe web preceding the calendering, for example a device provided with anextended calendering nip or a device provided with an extensiblecalendering nip and controlled in compliance with the load, favourably ashoe calender.

The porosity of the paper that is aimed at and most appropriatelymanufactured by means of the method and the paper machine in accordancewith the invention is higher than 150 Bendtsen units, measured incompliance with the Scan-P 60:87 standard, and the gloss is higher than25 Hunter gloss units, measured in compliance with the Tappi 4/80 M-90method. Characteristics of prior-art papers less well suited formulti-colour printing are, among other things:

basis weight: 80 . . . 200 g/m² (grams per sq.meter), typically about100 g/m²,

porosity: 150 . . . 1200 Bendtsen, typically 150 . . . 350 Bendtsen,

roughness: about 30 . . . 100 Bendtsen, typically about 50 Bendtsen,

thickness: 50 . . . 200 microns, typically about 100 microns,

gloss: 10 . . . 20 Hunter 75°,

non-coated, because coated papers are too dense, or

slightly (2 . . . 6 grams per sq.m per side) pigmented.

Thus, at present, no method is known for manufacture of a paper that isprovided with the combination of porosity/gloss provided by the presentinvention.

In connection with the present invention, all conventional raw-materialsof fine paper are suitable for use, both with short fibres and with longfibres, obtained from all wood species. As fillers, for example,carbonate and talc are used. Thus, the invention is carried into effectwith existing paper raw-materials known in themselves.

When a paper in accordance with the invention is manufactured, in a wayin itself known the additives, retention agents and fillers in the paperare applied as layers in the desired way from a multi-layer headbox tothe wire part, by means of which procedure attempts are made to providea chemical formation of layers in the Z-direction. When suitableadditives and fillers are used, there is almost no need to act upon thedistributions within the layers. Porosity of the paper can be producedin the wire part also by means of retention by using a retention agentand vacuums, whose use promotes the formation of porosity. However, itis also important that good formation is achieved. The former must besuch that it does not damage the layer formation that has been produced.For example, a gap former is suitable for this purpose, but formers ofother types are also suitable for use, at least to a limited extent. Inview of improving the gloss, the desired distribution of fillers issubstantially U-shaped. This form of distribution of fillers is obtainedby means of said formation of layers in the paper. Thus, in this way, abetter gloss is obtained after calendering.

In the press section, an extended-nip press is used, for example a shoepress. An extended nip is particularly well suited for this purpose,because in it the paper is compacted uniformly while the porosity isretained. Thus, in the Z-direction, a uniform distribution of density isobtained. If, for example, a roll press were used, the surface of thepaper would be compacted to a higher extent than the middle.

In drying, favourably impingement drying is employed, which maintainsthe porosity of the paper, but traditional cylinder drying is alsosuitable for use.

In surface sizing and pigmenting, the film transfer method is used, inwhich case the paper remains more porous. In such a case, a uniformlayer of coating agent is obtained, and the electric charge on the X/Yaxis is even. If necessary, the web can also be pre-calendered beforethe film coating.

As the calender, a calender is used that substantially retains theporosity of the web preceding the calendering, preferably a shoecalender, in which case the porosity of the paper is retained and thegloss and the smoothness can be brought to the desired levels. As anextended-nip calender, it is also possible to use a so-called beltcalender. With respect to a shoe calender and to a belt calender,reference is made to the applicant's Patent Application FI 973954 and tothe prior art referred to in said patent application. Also, for example,OptiLoad™ calenders and soft calenders are suitable for the arrangementin accordance with the invention, but their use is not equallyadvantageous as the use of a shoe calender.

If necessary, it is possible to use pre-calendering before coating, in away in itself known, in order to provide a low extent of pigmenting.

An important part in a paper machine in accordance with the invention isthe headbox and the wire part, by whose means the formation of layers isproduced, and a second important unit is the calender, by whose meansthe retaining of the porosity that has been achieved and the desiredgloss are secured. The porosity and the gloss and the other propertiesof the paper grade to be produced are, of course, also affected by meansof the surface sizing unit, the press section, and by means of thedrying method. It has, however, been noticed that the formation oflayers in the web and calendering of a correct type are the essentialfactors.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the figures in the accompanying drawing, the inventionbeing, however, not supposed to be strictly confined to the details ofsaid illustrations alone.

FIG. 1 is a schematic illustration of an exemplifying embodiment of thepaper machine in accordance with the present invention for carrying outthe method in accordance with the invention.

FIG. 2 is a schematic illustration of an embodiment of a calender for apaper machine in accordance with the invention.

FIG. 3 is a schematic illustration of a preferred solution of the shortcirculation for a paper machine in accordance with the invention.

FIGS. 4A-4D are schematic illustrations of a comparison of distributionof fillers.

FIG. 5 illustrates the porosity of paper as a function of the dry solidscontent of the paper when a roll press or a shoe press is employed.

FIGS. 6A-6D are schematic illustrations of the effects of blade coatingand film transfer coating in coating of paper.

FIG. 7 illustrates the permeability to air of coated paper afterdifferent coatings.

FIG. 8 illustrates the effect of calendering on the density of paper.

The advantageous exemplifying embodiment of the paper machine inaccordance with the present invention shown in FIG. 1, first, comprisesa headbox 100, which is most appropriately a multi-layer headbox inaccordance with what is shown in the figure. This is followed by thewire part 200. The headbox 100 and the wire part 200 have suchconstructions that, in the Z-direction, a structure of paper consistingof layers and/or the desired distribution of additives or fillers in theZ-direction is/are achieved (see the applicant's patent EP 0 651 092).When a retention agent suitable for the purpose and vacuums are employedin the wire part 200, the formation of porosity is promoted. It is,however, also important that good formation is achieved. The former 200must be such that it does not damage the layer formation that has beenproduced. The gap former 250 shown in the figure is well suited for thispurpose, but formers of other types are also suitable for use, at leastto a limited extent. In accordance with the figure, in the press section300, an extended-nip press is used, for example a shoe press 350,360,and in this way possibilities are created for retaining of the porosity.The press section 300 as shown in the figure comprises two presses350,360. A forward dryer section 400 is composed of an impingement dryer450 and of a conventional cylinder dryer 460, which comprises dryergroups R_(1 . . . N) which make use of single-wire draw. At least one ofthe dryer groups is composed of a large-diameter cylinder 420 placed inthe basement space and of an impingement drying equipment 422 fitted inconnection with said cylinder 420. After the forward dryer section 400,there is a film size press 500, which is followed by an after-dryer 600consisting of dryer groups R_(j1), R_(J2) that apply single-wire draw,as well as a shoe calender 700 and a reel-up 800.

In a paper machine as shown in FIG. 1, the paper web W runs as follows.Out of the multi-layer headbox 100 the stock is fed into the gap formedbetween the former rolls 210,220 of the gap former 250 in the wire part200, from which gap the web is passed, between the wires 215 and 216,over water drain devices 230 further, while supported by the wire 215,to the press section 300. The press section 300 comprises two presses350 and 360, and on the upper fabric 315 of the first press the web W ispassed to between the press rolls 311,310 of the press 350 whilesupported by the lower fabric 316. From the lower fabric 316 the web Wis passed onto the upper fabric 317 of the following press 360 andfurther, between the upper fabric 317 and the lower fabric 318, tobetween the press rolls 321,320 of the press 360. Each press 350,360 hasbeen formed as a shoe press. From the press section 300 the web W ispassed by means of a transfer fabric 390, while a suction box 391 keepsthe web W in contact with the fabric, to the impingement drying unit inthe dryer section 400, in which unit the web W runs on support of thelower fabric 451 over the impingement drying equipment 450 into thedryer groups R₁ . . . R_(N) with single-wire draw in the dryer section400. Of the cylinder dryer groups, the group R₂ has been formed suchthat it comprises a large-diameter cylinder 420 placed in the basement,in connection with which cylinder impingement drying 422 has beenarranged, in which dryer group the web runs on support of the wire 425.The drying wire of the dryer groups with single-wire draw is denotedwith the reference numeral 415, and the heated drying cylinders in theupper row with the reference numeral 410, and the reversing cylinders orrolls in the lower row with the reference numeral 411. The web W runsmeandering from the reversing cylinders/rolls 411 in the lower row ontothe heated drying cylinders 410 in the upper row, on which cylinders theweb W is in direct contact with the heated cylinder face. After this theweb W is passed through a measurement device 490 to a film size press500, whose rolls are denoted with the reference numerals 545 and 547,and whose film transfer means are denoted with the reference numerals548 and 549. Over a contact-free turning device 580 the web W is passedthrough an infra/airborne-web dryer 590 to an after-dryer section 600,which comprises two dryer groups R_(J1),R_(J2) which make use ofsingle-wire draw and which comprise drying wires 651 and heated dryingcylinders 610 as well as reversing cylinders/rolls 611. After theafter-dryer section, the web is moistened either with water mist or withsteam in view of elimination of possible curl by means of the device650. After that the web W is passed into a calender 700, which has beenformed as a shoe calender, and its rolls are denoted with the referencenumerals 750,751. After the calender 700, the web W is passed into areel-up, in which the paper web W is reeled on the reeling drums 809,810of the reel-up into paper reels 811,812.

FIG. 2 shows a shoe calender, in which an extended calendering nip N isformed between a hot hard roll 750 and a shoe roll 751. The shoe roll751 again comprises a press shoe 724 supported by a stationary beam 725as well as a calendering belt 720 passed around the press shoe 724 andthe beam 725 and formed as an endless loop. By means of the press shoe724, the necessary load is produced in the nip N.

FIG. 3 is a schematic illustration of a preferred solution of the shortcirculation, in which three stocks at different mixing ratios are passedinto the inlet headers in the multi-layer headbox 100 in view offormation of layers. From a mixing tank 111 the stock is passed into themachine tank 112, from which it is passed through a wire pit 113 andthrough removal of impurities 118 to deaeration 114, after which thestock is divided into three ducts, each of which has pumps 119 and 120of its own. Into the stock, retention agents, fillers and additives canbe passed in the desired ratios at three points 115,116,117 before thestock is passed into the headbox 100. In this way the desired layers ofcompositions are produced in the Z-direction of the web.

FIGS. 4A-4D illustrate distributions of fillers in different papers.FIGS. 4A-4C illustrate distributions of fillers in colour copyingpapers, in which in FIG. 4A the filler content is 8.8%, in FIG. 4B13.2%, and in FIG. 4C 8.7%. FIG. 4D illustrates a distribution offillers obtained with an arrangement in accordance with the invention ina test run, wherein the filler content was 20%, the weight 97.9 gramsper square meter, and the speed of manufacture was 18 meters per second.As comes out from FIG. 4D, the desired distribution of fillers isU-shaped. In FIGS. 4A-4D, the vertical axis represents the fillercontent as a percentage, and the horizontal axis represents thepercentage proportion in the basis weight of the paper.

FIG. 5 illustrates the porosity of paper obtained with different pressesas a function of the dry solids content. The vertical axis representsthe porosity as Bendtsen units, and the horizontal axis represents thedry solids content of the paper. The lower curve 31 has been producedwith a roll press, and the upper two curves 32,33 have been producedwith a shoe press used in connection with an arrangement in accordancewith the invention. As is seen from the figure, a shoe press isadvantageous in view of porosity. With the same dry solids content afterthe press, with a shoe press it is possible to obtain a considerablymore porous web than with a roll press; for example, in the case of FIG.5, when the dry solids content is 45%, the shoe press provides a paperwhose porosity is 300 Bendtsen units higher.

In FIGS. 6A-6D, blade coating, FIG. 6A, has been compared with coatingcarried out by means of the film transfer method, FIG. 6B, and, as comesout from FIGS. 6C-6D, with the film transfer method more even coatinglayers 541 are provided on the paper 542. In FIG. 6A, the roll isdenoted with the reference numeral 543 and the blade coater with thereference numeral 544, the paper web 542 that passes by being coated bymeans of said blade coater 544.

FIG. 6B shows a film transfer equipment 500, in which the rolls aredenoted with the reference numerals 545 and 547, and by means of saidrolls 545,547 coating agent is transferred from the coating device546,548 onto the face of the paper web 542.

In FIG. 7, the vertical axis represents the permeability to air of paperwith different coating procedures: columns 51A-51C a short-dwell coater,columns 52A-52C a blade coater, columns 53A-53C a nozzle applicator, andcolumns 54A-54C a film transfer method. The letter A refers to 0% DIP, Bto 40%, and C to 60%. As comes out from the figure, the film transfermethod provides the best porosity, which comes from a more uniform layerof coating agent and from less oriented particles. In the test, offsetpaper of 58 g/m² was used, the weight of the coating was 8 g/m². Thevertical axis represents the Curley-Hill penetrability to air, and theunit is seconds per 100 milliliters.

FIG. 8 illustrates a comparison of calendering, and the horizontal axisrepresents the linear load, the unit being kN/m, and the vertical axisrepresents the resistance to air, the unit being seconds per 100milliliters. The moisture content of the paper used in the test beforecalendering was 4.1-4.7%, and the ultimate moisture content was3.2-4.1%. As comes out from FIG. 8, when shoe calendering was used(curves OptiDwell Shoe 1 . . . 2 nips, temperatures 160/200° C.) therewere no losses in porosity.

Above, the invention has been described with reference to some preferredexemplifying embodiments of same only, the invention being, however, byno means supposed to be strictly confined to the details of saidembodiments alone.

What is claimed is:
 1. A method for manufacturing glossy and porouspaper comprising the steps of: preparing a plurality of stock flows eachof said stock flows; selectively adding to each of said stock flowsretention agents, fillers and additives to promote the glossy and porousnature of the paper; passing each of said stock flows to a headbox;passing each of said stock flows from said headbox onto a wire of a wirepart in layers to produce a web having a plurality of layers arranged ina Z direction to promote the glossy and porous nature of the paper, eachone of said layers having a selected distribution of additives andfillers; feeding said web from said wire part to a press section;feeding said web from said press section to a dryer section; feedingsaid web from said dryer section to a coating section and coating saidweb in said coating section; feeding said web from said coating sectionto an after-dryer section; feeding said web from said after-dryersection to a calender section; and feeding said web from said calendersection to a reel-up; and calendering said web in at least onecalendering nip such that a porosity of said web is maintainedsubstantially equal to a porosity of said web prior to said calenderingnip wherein said paper has a porosity from about 200 to about 1200Bendtsen units and wherein said paper has a gloss higher than 20 Hunter75°.
 2. A method according to claim 1, wherein said calendering stepcomprises calendering said web in a shoe calender.
 3. A method accordingto claim 1, further comprising arranging a gap former in said wire part.4. A method according to claim 1, further comprising arranging a vacuumin said wire part and wherein said retention agents and said vacuum areemployed for promoting a porosity of said web.
 5. A method according toclaim 1, wherein said calendering step comprises coating said web bymeans of the film transfer method.
 6. A method according to claim 1,further comprising pressing said web in said press section by means of aextended-nip to retain a porosity of said web.
 7. A method according toclaim 1, wherein said headbox is a multilayer headbox.
 8. A methodaccording to claim 1 further comprising drying said paper web in saiddryer section by impingement drying.
 9. A method according to claim 1,wherein said fillers are added to said stock flows and said stock flowsare passed from said headbox to said wire part such that a U-shapeddistribution of fillers is formed into said paper web.
 10. A paperaccording to claim 1, wherein said paper has a porosity from about 200to about 500 Bendtsen units.
 11. A paper according to claim 10, whereinsaid paper has a gloss higher than 25 Hunter 75°.
 12. A paper machinefor the manufacture of glossy and porous paper, said machine comprising:a headbox; a wire part; a press section; a dryer section; a coatingsection; an after dryer section; a calender; and a reel up; wherein saidheadbox and said wire part are structured and arranged to form a paperweb having a plurality of layers in a Z direction to promote the glossyand porous nature of the paper, said headbox and said wire part beingstructured and arranged so that each one of said layers has a selectedcomposition; a short circulation section arranged before said headboxfor producing a plurality of stock flows, and wherein said shortcirculation section includes means for selectively adding fillers andadditives to each of said stock flows to promote the glossy and porousnature of the paper; and wherein said calender comprises a calenderdevice structured and arranged to substantially maintain a porosity ofsaid paper web after said calender device at a level substantially equalto a porosity of said paper web prior to said calender device whereinsaid paper has a porosity from about 200 to about 1200 Bendtsen unitsand wherein said paper has a gloss higher than 20 Hunter 75°.
 13. Thepaper machine according to claim 12, wherein said calender deviceincludes an extended calendering nip.
 14. The paper machine according toclaim 13, wherein said calender device is controlled in compliance withloading and provided with a extensible calendering nip.
 15. The papermachine according to claim 12, wherein said calendering device is a shoecalender.
 16. The paper machine according to claim 12, wherein saidcoating section includes means for coating said web by the film transfermethod.
 17. The paper machine according to claim 12, wherein said presssection comprises at least one extended-nip press.
 18. The paper machineaccording to claim 12, wherein said wire part includes a former and saidformer is a gap former.
 19. The paper machine according to claim 12,wherein said headbox is a multi-layer headbox.
 20. The paper machineaccording to claim 12, wherein said dryer section includes at least oneimpingement drying unit.
 21. A method for manufacturing glossy andporous paper comprising the steps of: preparing a plurality of stockflows each of said stock flows; selectively adding to each of said stockflows retention agents, fillers and additives to promote the glossy andporous nature of the paper; passing each of said stock flows to aheadbox; passing each of said stock flows from said headbox onto a wireof a wire part in layers to produce a web having a plurality of layersarranged in a Z direction to promote the glossy and porous nature of thepaper, each one of said layers having a selected distribution ofadditives and fillers; feeding said web from said wire part to a presssection; feeding said web from said press section to a dryer section;feeding said web from said dryer section to a coating section andcoating said web in said coating section; feeding said web from saidcoating section to an after-dryer section; feeding said web from saidafter-dryer section to a calender section; and feeding said web fromsaid calender section to a reel-up; and calendering said web in at leastone calendering nip such that a porosity of said web is maintainedsubstantially equal to a porosity of said web prior to said calenderingnip wherein said paper has a gloss higher than 20 Hunter 75°.
 22. Apaper produced by the method according to claim 21, wherein said paperhas a porosity from about 200 to about 1200 Bendtsen units.